Islet Isolation
The insulin producing tissue of the pancreas, the islets of Langerhans, constitutes between about one and two percent of the mass of the pancreas. The isolation of the islets from the remainder of the pancreatic tissue is desirable for laboratory purposes and for transplantation purposes. Transplantation of islets is looked to as a possible treatment for diabetes. Transplanting islets rather than an intact pancreas or pancreatic segments offers several advantages, including the ease of transplantation, the possibility of in vitro treatment to prevent rejection without immunosuppression, the elimination of the pancreatic exocrine function (the secretion of digestive substances from the host tissue), the possibility of cryopreservation of the tissue for subsequent use, and the possibility of xenografts.
In an early method of islet separation, chopped pancreatic fragments are mixed with collagenase and incubated at 37.degree. C. (reviewed in Scharp, World Journal of Surgery 8:143-151 (1984)). The collagenase breaks down or digests the pancreatic tissue, freeing the islets. The collagenase also acts on the islets, so that the islets released early in the process are broken down into single cells. If the process is stopped to protect the islets released early, many islets remain trapped in pancreatic fragments. Therefore only a fraction of the available intact islets are released by this method. This process is particularly ineffective for the isolation of islets from the pancreata of large animals such as humans, dogs, or pigs.
Laboratory islet isolation from rodent pancreata was greatly improved by the discovery that mechanical distension of rodent pancreata increased islet yield by causing mechanical separation of islets from the pancreas tissue. After distension the pancreas is chopped for coIlagenase digestion. The beneficial effect of this same type of mechanical distension has also been noticed in large animals.
Horaguchi and Merrell, Diabetes 30:455-58 (1982) developed a method for perfusing the dog pancreas with collagenase via the pancreatic duct. Subsequently, a process involving ductal distension of the pancreas with a solution containing collagenase was developed (U.S. Pat. No. 4,868,121; incorporated herein by reference). Inflation or distension of the pancreas is believed to cause some mechanical rupturing of the exocrine tissue or partial separation of the islets from the exocrine tissue, making subsequent collagenase digestion easier.